Send the play queue ID to the server

The server keeps a play queue per client, identified by the X-Queue-Id
header. The client sends the ID it has for the selected server with every
request and keeps the one in each response, which is new after a reset or a
sync. The IDs are saved per server in the [queues] section of settings.cfg,
so the game goes on with the same queue after a restart.

HttpTransport now passes the response headers on to the client.

server/http_transport.gd

@@ -16,8 +16,9 @@ func _init(owner: Node) -> void:
 
 
 ## Requests [param url] and calls [param on_complete] with
-## [code](response_code: int, body: PackedByteArray)[/code]. Connection errors and
-## timeouts are reported with response code 0. [param headers] are sent as-is; a
+## [code](response_code: int, body: PackedByteArray, headers: PackedStringArray)[/code],
+## the headers as [code]"Name: value"[/code] lines. Connection errors and timeouts are
+## reported with response code 0 and no headers. [param headers] are sent as-is; a
 ## [param body] is sent as JSON.
 func request(
 		url: String,
@@ -33,12 +34,14 @@ func request(
 		func(
 				result: int,
 				response_code: int,
-				_headers: PackedStringArray,
+				response_headers: PackedStringArray,
 				response_body: PackedByteArray,
 		) -> void:
 			http_request.queue_free()
-			on_complete.call(response_code if result == HTTPRequest.RESULT_SUCCESS else 0,
-					response_body)
+			if result != HTTPRequest.RESULT_SUCCESS:
+				on_complete.call(0, response_body, PackedStringArray())
+				return
+			on_complete.call(response_code, response_body, response_headers)
 	)
 	var all_headers: PackedStringArray = headers.duplicate()
 	if method == HTTPClient.METHOD_PUT || !body.is_empty():
@@ -47,4 +50,4 @@ func request(
 	if error != OK:
 		http_request.queue_free()
 		push_error("Could not send request to %s (error %d)" % [url, error])
-		on_complete.call(0, PackedByteArray())
+		on_complete.call(0, PackedByteArray(), PackedStringArray())

server/music_server_client.gd

@@ -6,6 +6,11 @@ extends Node
 ## goes through [member transport]; tests replace it with a fake to answer requests
 ## without a running server.
 ##
+## The server keeps a play queue for each client, identified by the [code]X-Queue-Id[/code]
+## header. The client sends the ID it has for the server with every request and keeps the
+## one in each response, which is new when the server started a new queue (after a reset
+## or a sync). The IDs are kept in [code]Settings.queue_ids[/code], which saves them.
+##
 ## Requests need a token, which the client gets with [code]Settings.api_password[/code]
 ## the first time it is needed and then reuses. When the server answers 401 (the token
 ## expired, or the server restarted), it gets a new token and sends the request once
@@ -21,6 +26,8 @@ extends Node
 const API_PATH: String = "/api/v1"
 ## Sent when asking for a token, so the server can tell its clients apart.
 const CLIENT_TYPE: String = "godot"
+## Header with the ID of the client's play queue, in requests and responses.
+const QUEUE_HEADER: String = "X-Queue-Id"
 
 ## Sends the actual HTTP requests. Swap it out in tests.
 var transport: HttpTransport
@@ -301,6 +308,25 @@ func clear_token() -> void:
 	_token_server = ""
 
 
+# Queue IDs
+
+## The ID of the play queue on the selected server, or "" before the server has sent one.
+func queue_id() -> String:
+	return Settings.queue_ids.get(Settings.default_path, "")
+
+
+# Keeps the queue ID in [param headers] from [param server] when it is new
+func _remember_queue_id(server: String, headers: PackedStringArray) -> void:
+	var prefix: String = QUEUE_HEADER.to_lower() + ":"
+	for header: String in headers:
+		if !header.to_lower().begins_with(prefix):
+			continue
+		var id: String = header.substr(prefix.length()).strip_edges()
+		if !id.is_empty() && Settings.queue_ids.get(server, "") != id:
+			Settings.set_queue_id(server, id)
+		return
+
+
 # Generic requests
 
 ## Requests [param path] and calls [param on_done] with the parsed JSON.
@@ -415,11 +441,19 @@ func _send_once(
 func _send(
 		path: String, method: int, token: String, on_complete: Callable, body: String = "",
 ) -> void:
+	var server: String = Settings.default_path
 	var headers: PackedStringArray = []
 	if !token.is_empty():
 		headers.append("Authorization: Bearer " + token)
-	transport.request(Settings.default_path + API_PATH + path, method, on_complete, headers,
-			body)
+	var queue: String = Settings.queue_ids.get(server, "")
+	if !queue.is_empty():
+		headers.append("%s: %s" % [QUEUE_HEADER, queue])
+	transport.request(server + API_PATH + path, method,
+			func(response_code: int, response_body: PackedByteArray,
+					response_headers: PackedStringArray) -> void:
+				_remember_queue_id(server, response_headers)
+				on_complete.call(response_code, response_body)
+	, headers, body)
 
 
 func _handle_response(